Large area integrated rf amplitude modulators comprising series and shunt connected varactors have been proposed previously. See a commonly assigned United States patent of Smith et al., which issued Nov. 1, 1988 as U.S. Pat. No. 4,782,350 on "Amorphous Silicon Varactors as RF Amplitude Modulators and Their Application to Acoustic Ink Printing." That patent hereby is incorporated by reference, especially for its description of acoustic ink printing and its description of several of the different configurations in which such varactors may be employed to function as rf amplitude modulators for acoustic ink printheads.
However, there still is a need for large area integrated varactors which have relatively faithful rf frequency responses and which can be relatively rapidly switched back and forth between their maximum and minimum capacitance levels, C.sub.max. and C.sub.min., respectively to provide increased capacitive switching ratios, R, where: EQU R=C.sub.max. /C.sub.min. ( 1)
For example, for acoustic ink printing and the like, it would be desirable to have large area integrated varactors having capacitive switching ratios, R, of approximately two (2) or more for amplitude modulating rf voltages. Taking this example a step further, the rf voltages used for acoustic ink printing typically have peak voltage levels of roughly 60 volts or so and frequencies on the order of about 100-200 MHz. Moreover, printing conventionally is performed by amplitude modulating such voltages at data rates on the order of about 10 Khz in accordance with input data sample pulses having duty ratios of up to approximately 10% or so.